Using Analytics goal tracking for 3 products leading to the same thank-you page

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I've setup a lead gen site that has 3 products.

All are courses/webinar type products where the form completion leads to the same thank-you page.

Because of the booking plugin I'm using, I'm restricted to having only the single thank-you page.

In Google Analytics I've setup goal funnels for each of the products, but because they all lead to the same thank-you page, each lead is counted 3 times - one time for each product, royally messing up stats.

How do I do this? How do I count register only the correct product lead?